1. Field of the Invention
This invention is directed generally to radio communication systems, and more particularly to improved linearity in radio communication systems that use Gilbert mixers.
2. Description of the Related Art
In radio communication systems, a mixer is used to up-convert a baseband signal to a higher frequency (e.g., radio frequency (RF)) signal for ease of transmission. The mixer can also down-convert a high frequency signal to baseband for ease of signal processing. Various types of mixers exist, including unbalanced, single and double balanced, and the four-quadrant or Gilbert mixer. For general information regarding the various types of mixers, the reader is referred to “Radio-Frequency Microelectronic Circuits for Telecommunication Applications,” Yannis E. Papananos, ISBN 0-7923-8641-8, Kluwer Academic Publishers, Boston, 1999.
The Gilbert mixer is commonly used because this mixer design provides reasonable conversion gain (i.e., intermediate frequency (IF) output power with respect to RF input power), good image rejection at the RF and local oscillator (LO) ports, and a differential IF output. FIG. 1 illustrates a circuit diagram for an exemplary Gilbert mixer 100 that can be used to down-convert a high frequency (e.g., RF) signal. As can be seen, the Gilbert mixer 100 is composed of two main sections: an amplifier 102 for receiving and amplifying the RF signal, and a mixer core 104 for mixing the RF signal with the LO signal to produce the IF signal.
The amplifier 102 includes two transistors Q1 and Q2 connected as a typical single stage (i.e., one transistor stage) amplifier. The source terminal of each transistor Q1 and Q2 is connected to a common ground Vss via a respective one of the feedback resistors RE. The gate terminals of the two transistors Q1 and Q2 form the RF input port 106 through which the RF input signal is received. The RF input signal is then provided to the mixer core 104 via the drain terminals of the two transistors Q1 and Q2. The above single stage arrangement is also known as a “local feedback” arrangement because of the direct influence of the transistor output ie on the transistor input vb:vb=vbe+ieRE  (1)
The mixer core 104 includes fours transistors Q3–Q6 connected as a typical mixer circuit. The source terminals of transistors Q3 & Q4 are connected to the drain terminal of transistor Q1. Likewise, the source terminals of transistors Q5 & Q6 are connected to the drain terminal of transistor Q2. The gate terminals of transistors Q4 & Q5 are connected together and form one end of the LO input port 108. The other end of the LO input port 108 is formed by the common gate terminals of transistors Q3 & Q6. The drain terminals of transistors Q3 & Q5 and Q4 & Q6 together form the IF output port 110. Pull-up resistors R connect the drain terminals of the transistors Q3–Q6 to the power supply.
Operation of the Gilbert mixer 100 is as follows. In the absence of any voltage difference between the gates of transistors Q1 and Q2, the drain currents ic of these two transistors are essentially equal. Thus, a voltage applied to the LO input port 108 results in no difference at the IF output port 110. Should a small DC offset voltage be present at the RF input port 106 (e.g., due to a mismatch in the transistors Q1 and Q2), this will only result in a small feed through of the LO signal to the IF output port 110, which will typically be blocked by an IF filter (not shown). Conversely, if an RF signal is applied to the RF input 110 port 106, but no voltage difference is applied to the LO input port 108, the IF output port 110 will again be balanced. A small offset voltage (due to mismatch in transistors Q3–Q6) may cause some RF signal feed through to the IF output port 110. As before, however, this will be rejected by the IF filter. Thus, it is only when a signal is supplied to both the LO input port 108 and the RF input port 106 that a signal appears at the IF output port 110.
A problem with Gilbert mixers in general is that the amplifier 102 does not achieve a sufficiently high range of linearity for modern radio communication systems. FIG. 2 is an exemplary graph showing the relationship between the RF input voltage (VRF) and output voltage (VIF) for the Gilbert mixer 100. As can be seen, there is essentially a linear relationship between the VRF and the VIF signals for a certain operating range, generally between points 200 and 202. This response is due primarily to the linear transconductance of the amplifier 102 over that operating range. Outside this operating range, however, the Gilbert mixer 100 becomes increasingly nonlinear. The size of the linear operating range depends mostly on the operating points of Q1 and Q2 as well as the feedback provided to the amplifier 102. Generally, an increase in the amount of feedback results in an increase in the linearity of the mixer. In FIG. 1, for example, the resistors RE provide feedback to the amplifier 102. Increasing the resistance of the feedback resistors RE will have the effect of increasing the linearity of the Gilbert mixer 100.
While the above described Gilbert mixer design can provide sufficient linearity for older systems, modern radio communication systems such as UMTS (Universal Mobile Telecommunications System) have much greater linearity requirement due to the newer modulation techniques used in combination with a reduced supply voltage. That is, modern radio communication systems such as UMTS require a larger linear operating range in the mixer. This increased linearity requirement is stretching the capability of most existing Gilbert mixer designs. Specifically, the local feedback arrangement of the single stage amplifier of existing Gilbert mixer designs cannot provide sufficient feedback to produce the linearity required by some modern radio communication systems when low supply voltages are employed.
Therefore, it is desirable to provide a Gilbert mixer that is capable of performing with a higher linearity than that of existing Gilbert mixers. In particular, it is desirable to provide a Gilbert mixer having an improved amplifier feedback that can produce the increased linearity required by modern radio communication systems.